A far more interesting question is: where do you draw the line between "Islamist extremism"and "traditional religious practice"?
To take an obvious example, if you are giving a sermon in a mosque somewhere and quote that line from the Quran about unbelievers being the worst of people, is that "hate preaching" (arguably, it certainly is) or is it simply "traditional religious practice"?
It can easily be argued both ways, depending on your preference.
I can see a distinction. If all they say is that "atheists are smelly dirty pigs and will go to hell" it is an empty threat to me, so that would come under Religious Traditional Practice. Its offensive and I dislike it, but not scared or threatened. Should be legal.
If they publically say "atheists must be killed" then that is an incitement to commit murder and should be a crime and will need a talking to. If they then urge a 'call to action' to murder atheists then they should be arrested and bought to justice. Should be illegal.
So to summarize simply, I would say divine threats should be legal (ie god thinks you smell and will kill you) but real threats (ie WE will/should/must kill you) are illegal.
I am sure there are some grey areas but just a broad definition from me.